Heretofore, various resins have been synthesized which have double bonds in the molecule and can form crosslinking of network structure through the cleavage of the double bonds on exposure to electron radiation. On the other hand, it has been found that a polyester resin having hydrophilic sulfonate groups but having no double bonds in the molecule is good in ability to disperse magnetic particles when used as a binder for a magnetic coating material for producing magnetic recording medium such as magnetic tape, but is poor in solvent resistance and mechanical strength of the coating film when applied to magnetic tape. Moreover, it has been found that a polyester-polyurethane resin having metal sulfonate groups but having no double bonds in the molecule is also good in ability to disperse magnetic particles and abrasion resistance of coating film but is not satisfactory in solvent resistance.